Laser absorption spectroscopy is a common method of measuring and identifying chemicals. For example, light from an Er:YAG laser operating at a wavelength near 1645 nm is strongly absorbed by methane; the reduction in laser intensity, due to absorption, can be used to calculate the amount of methane probed by the laser beam. The laser source is more effective if its spectral bandwidth is limited; a spectrally pure laser is more useful than one that is not as pure. In addition, it is often easier to measure the absorption signal in a string of short pulses instead of in a long pulse or a continuous laser. A stable, spectrally pure laser output with a high repetition rate of short pulses is ideal. There can be a tradeoff between short pulses and spectral purity.
There are many applications for high-quality laser absorption spectroscopy. The methane absorption at 1645 nm is already used by Er:YAG lidar systems to measure methane concentration in air. Likewise, the carbon dioxide absorption near 2100 nm has been used by Ho:YAG lidar systems to measure carbon dioxide concentration in the air. Laser absorption spectroscopy is also used to measure, for example, pollutants dissolved in water.
Due to the effects of climate change, there is an increasing need to measure greenhouse gases accurately. These include carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons. Monitoring these gases, and finding sources of leaks of the gases, is a key application of laser absorption spectroscopy. As one example, a laser mounted in an aircraft and pointed to the ground can be used as the source; the reflected signal, which can then be captured by a measurement system mounted on the same aircraft, passes through a column of atmosphere twice, creating a long absorption path that can enhance absorption of the laser beam and increase measurement accuracy.
There are several laser absorption spectroscopy systems under development to measure greenhouse gases, atmospheric pollutants, and chemicals dissolved in the oceans. Most such systems can only measure a single chemical, although some, such as those that use an optical parametric oscillator, can be tuned enough to potentially measure more than one chemical. The shortest laser pulses typically used for these measurements is 7-10 ns, and the spectral bandwidth of the laser is typically larger than a single absorption feature.